1. Field of the Invention
The present invention relates to a luminous ink composition which emit light when exposed to ultraviolet rays, specifically relates to a blue luminous ink composition.
2. Description of the Related Art
In recent years, the term xe2x80x9cfunctional organic compoundxe2x80x9d has come to be employed and organic substances for use in electronic or optical devices are actively studied. Of such substances, luminous compounds showing a photoluminescence (PL) phenomenon are known.
One application of the luminous compounds is security ink. The security ink is an ink such that a handwriting is invisible under visible light, however it becomes visible under irradiation of specific light or the handwriting emits light when it is irradiated with ultraviolet rays, e.g., a black light lamp, and one can read the recorded information.
The security ink is used for the purpose of preventing forgery or reproduction or of recording secret information. For instance, if lot numbers or codes of items are recorded with a security ink, it becomes easy to trace the distribution channels of the items or to prevent production or distribution of counterfeit products, and the fear of manipulation or damage of recorded information is reduced because the information is invisible under visible light.
Conventionally, a red luminous ink composition having europium complex has been mainly proposed as a security ink of this type, as described in Japanese Patent Kokai Publication No. 2000-144029, and No. 2000-160083 and the like.
In Japanese Patent Kokai Publication No. 2000-144029, proposed is an ink composition with the following features: being substantially invisible within the visible light region; containing europium (Eu) complex having an emission center wavelength at 615xc2x120 xcexcm when being excited with ultraviolet rays; containing a polyvinyl resin; containing, as a neutral ligand, at least a phosphoric acid type compound selected from a phosphine oxide compound, a phosphine sulfide compound and a phosphine compound; and containing not less than 94% of water and/or ethanol in a solvent.
In Japanese Patent Kokai Publication No. 2000-160083, proposed is an ink composition for ink jet printing containing, as a luminous material, from 0.001 to 5% by weight of 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionate europium chelate compound and also containing not less than 70% by weight of an alcohol solvent as a solvent.
On the other hand, as to a blue luminous ink composition, a fluorescent brightening agent has been generally used for a luminous compound. For example, Japanese Patent Kokai Publication No. 2000-248219 describes a hot-melt type stealth ink composition which employs a fluorescent brightening agent xe2x80x9cUVITEX-OB (trade name)xe2x80x9d available from Ciba Specialty Chemicals Ltd (example 2).
However, the conventional rare-earth metal complexes which has beta-diketones as a ligand are rather expensive due to the presence of rare-earth metal, and are not suitable for practical use.
The inventors of the present invention have investigated into an ink composition containing a metal complex (e.g., aluminium complex, or indium complex) having specific beta-diketones as a ligand. As a result, it has been found that a blue luminous ink composition which shows fluorescent strength, visibility and fastness high enough to practical use is available by employing the metal complex, and the present invention has been completed.
The present invention provides a blue luminous ink composition comprising the luminous compound represented by the formula: 
wherein R1 is a group selected from the group consisting of a hydrogen atom, an alkyl group having from 1 to 4 carbon atoms, an alkoxyl group having from 1 to 4 carbon atoms and a halogen atom, R is a group selected from the group consisting of a fluorine-comprising alkyl group having from 1 to 20 carbon atoms, a substituted or unsubstituted alkyl group having from 1 to 4 carbon atoms, a substituted or unsubstituted aralkyl group having from 7 to 10 carbon atoms and a substituted or unsubstituted aryl group having from 6 to 12 carbon atoms, M is a two-valent or three-valent metal, provided that rare-earth metal and alkaline earth metal (IIa group) are eliminated, and n is an integer of 2 or 3.
The blue luminous ink composition is substantially invisible under visible light, however it shows intense emission colour within the blue region under ultraviolet rays. Therefore, the blue luminous ink composition is useful for printing a blindfold mark, bar-code or the like on office documents and various containers.